The identification of DC-SIGN as a key mediator of dendritic cell transmission of HIV to CD4+ T cells has provided an important molecular insight into a previously enigmatic process. HIV inoculation of primary dendritic cells cocultured with CD4+ T cells leads to a rapid and fulminating infection. In contrast, infection of either cell population individually results in a more limited proliferation of virus. DC-SIGN, a C-type lectin receptor highly expressed on immature myeloid derived dendritic cells, directly interacts with the HIV particle and is a major participant in stimulating HIV infection of CD4+ T cells. A striking aspect of this process is that DC-SIGN enhancement of HIV infection occurs with remarkable efficiency in trans. Although it is not currently understood how the trans donation of HIV from donor to target cell takes place, DC-SIGN mediated HIV transmission can be reconstructed with transformed cell lines facilitating examination of the underlying mechanism. We have extended this analysis and examined different cell types for their ability to support DC-SIGN mediated HIV transmission. Surprisingly, only restricted cell types expressing DC-SIGN support HIV transmission at levels comparable to primary dendritic cells. Thus, we propose experiments to understand how cell factors influence the DC-SIGN mediated transmission of HIV. These studies have also compelled us to look more carefully at HIV transmission by the DC-SIGN related molecule L-SIGN/DC-SIGNR. In the cell types examined, L-SIGN transmission of HIV was substantially reduced relative to DC-SIGN expressed in the same cells or primary dendritic cell transmission of HIV. This difference in HIV transmission by DC-SIGN and L-SIGN has allowed us to construct chimeric molecules between the two in the hope of identifying domains within DC-SIGN that contribute to HIV transmission after particle capture. DC-SIGN interactions with HIV are likely to be relevant to viral pathogenesis in vivo, and may also contribute to mucosal transmission of HIV. To aid in our understanding of these in vivo processes, we propose to screen for compounds that impair DC-SIGN interactions with HIV, and test for the effects of these compounds in animal models. We also propose to characterize a newly identified DC-SIGN family molecule in rhesus macaque. Although our interest in DC-SIGN stems from its interactions with HIV, DC-SIGN has been identified as a receptor for a number of significant human pathogens. Thus examination of DC-SIGN function in HIV infection may also contribute to our ability to intervene with other assaults on the immune system that occur via DC-SIGN.